Field of the Invention
The present invention relates to a polymer electrolyte membrane, a method for producing the same, and a membrane electrode assembly including the same.
Description of Related Art
Fuel cells are electrochemical cells capable of directly converting chemical energy that is generated by oxidation of fuel to electric energy, and fuel cells have been highly expected as the next-generation energy sources due to their environment-friendly features such as high energy efficiency and reduced emission of contaminants.
Generally, a fuel cell has a structure in which an electrolyte membrane is interposed between an oxidation electrode (anode) and a reduction electrode (cathode), and such a structure is referred to as a membrane electrode assembly (MEA).
Fuel cells can be classified into alkaline electrolyte fuel cells, polymer electrolyte membrane fuel cells (PEMFC), and the like, and among them, polymer electrolyte membrane fuel cells are attracting much attention as portable, automotive and domestic power supply devices due to the advantages such as a low operating temperature of lower than 100° C., rapid starting and responding characteristics, and excellent durability.
A representative example of such polymer electrolyte membrane fuel cells is a proton exchange membrane fuel cell (PEMFC) that uses hydrogen gas as a fuel.
To briefly describe the reaction occurring in a polymer electrolyte membrane fuel cell, first, when a fuel such as hydrogen gas is supplied to an anode, protons (H+) and electrons (e−) are produced by an oxidization reaction of hydrogen at the anode. The protons (H+) thus produced are delivered to a cathode through a polymer electrolyte membrane, and the electron (e−) thus produced are delivered to the cathode through an external circuit. Oxygen is supplied to the cathode, and oxygen binds to the protons (H+) and electrons (e−) so that water is produced as a result of a reduction reaction of oxygen.
Since the polymer electrolyte membrane is a channel through which protons (H+) produced at the anode are delivered to the cathode, basically, the polymer electrolyte membrane should have excellent conductivity for protons (H+). Furthermore, a polymer electrolyte membrane should have excellent separation performance of separating protons that are supplied to the anode and oxygen that are supplied to the cathode, and in addition to that, a polymer electrolyte membrane should have excellent mechanical strength, dimensional stability, chemical resistance, and the like. Also, a polymer electrolyte membrane needs to have characteristics such as a small ohmic loss at a high current density.
One class of the fluororesins that are currently used for polymer electrolyte membranes is perfluorosulfonic acid resins (hereinafter, referred to as “fluorine-based ion conductors”). However, fluorine-based ion conductors have weak mechanical strength, and thus have a problem that when the fluorine-based ion conductors are used for a long time period, pinholes are generated, and therefore, the energy conversion efficiency is decreased. There have been attempts to use a fluorine-based ion conductor having an increased membrane thickness, in order to intensify the mechanical strength; however, in this case, there is a problem that the ohmic loss is increased, expensive materials should be used in larger quantities, and thus economic efficiency is decreased.
In order to ameliorate the disadvantages of the fluorine-based ion conductors such as described above, development of hydrocarbon-based ion conductors has been actively conducted in recent years. However, because a polymer electrolyte membrane repeatedly undergoes expansion and contraction in a wet or dry state, which is a condition for operation of a fuel cell, hydrocarbon-based polymer electrolyte membranes that have high percent water contents due to their structures have a disadvantage that the long-term durability of the membrane is poor due to low dimensional stability and low tensile strength.
In order to solve such problems, there has been proposed a polymer electrolyte membrane in the form of a reinforced membrane in which mechanical strength has been enhanced by introducing a support as a reinforcing agent into the hydrocarbon-based ion conductor. Regarding the support, a non-ion-conductive hydrophobic hydrocarbon-based polymer support is mainly used. When such a hydrophobic support is used, dimensional stability is improved, and as a result, mechanical properties such as tensile strength can be secured even after the ion conductor has absorbed moisture, while the film thickness can be minimized for the purpose of minimizing the membrane resistance and enhancing the performance.
On the other hand, in order to produce the hydrocarbon-based ion conductor into a reinforced film form, an impregnation solution is prepared by dissolving the hydrocarbon-based ion conductor in a solvent, and then a method of immersing the porous support in the impregnation solution for a certain time period, or applying the impregnation solution on the surface of the porous support is used. However, in the case of the method described above, if the support has low impregnatability, or during the process of removing the solvent by evaporating the solvent after the impregnation or coating step described above, the affinity between the hydrocarbon-based ion conductor and the porous support may be decreased, and defects such as cavities may occur in the interior of the porous support. Then, due to the phenomenon in which the film of the relevant portion is pressed down by such cavities, cracking, membrane-electrode detachment, and the like may occur. Therefore, the impregnation or coating step is repeated several times, and accordingly, the thickness of the polymer electrolyte membrane is increased, while the thickness becomes non-uniform.
Furthermore, in the case of using a support with low porosity, there is a problem that the battery performance is deteriorated because the support itself serves as a resistance, and there occurs a problem of deterioration of the battery performance. In this regard, a reinforced membrane in which a support having a nanoweb structure with a maximized porosity has been proposed. However, despite its excellent performance and physical properties, such a reinforced membrane exhibits a reduction in performance under low-humidified operating conditions (less than 60%), rather than under high-humidified operating conditions (60% to 100%).
Therefore, there is a high demand for a technology that can realize minimization of the resistance and maximization of the battery performance at the time of production of a reinforced membrane containing a hydrocarbon-based ion conductor, by increasing the impregnatability of the hydrocarbon-based ion conductor into the porous support to the extent that does not affect the dimensional stability.